As an example, a golf player conventionally estimates the distance from his or her position to a flagstick (also known as a pin) by visual observation on a golf course and selects a golf club suitable for the distance. Since the selection of golf clubs significantly affects the player's scores, it is highly desirable to know the exact distance to the pin. Among known examples of the prior art, Japanese Patent Application Publication No. 60-95374 discloses a range-finding system comprising a portable transmitter which transmits an audible or ultrasonic sound signal and a solar cell powered receiver-transmitter (or a responder) which, mounted on a pin as a substitute for the flag, returns a response signal upon receiving the signal from the transmitter. The transmitter carried by a player measures the time delay from the transmission of the signal to the reception of the response signal and calculates the target distance. This system enables the player to know the exact distance to the pin by operating the transmitter.
In the above-described range-finding system, the capacity of the responder's power supply is considerably limited by its own weight and volume because the responder should be mounted on the pin. It is therefore impossible to operate the system so frequently even if the power supply is associated with solar cells for charging. Another drawback of the system is that players who have been accustomed to ordinary flagsticks could be embarrassed as they look at the responder mounted on the pin in place of the flag. Furthermore, since the mounting height of the responder is limited by the height of the pin, it is likely that signal exchanges between the transmitter and responder are difficult or limited in range if the golf course has prominent ups and downs.
As seen above, the conventional range-finding system is designed to allow measurement of the exact distance from the player's position to the target object, but the system has many problems because the responder is mounted on the pin. This means that the conventional system cannot be used in a case where it is impossible to mount the responder on the target object, not only at a golf course but also in other applications.